As Internet is becoming widely used and the environment where software is freely shared is popular, a variety of software viruses increasingly spread to invade stored information. For example, files in a computer or emails downloaded via personal digital assistants (PDAs) or mobile phones are likely to be damaged by software viruses. In order to reduce the damage of virus infection, a typical way is to employ virus-scanning software to detect and remove viruses. Such virus-scanning software is generally installed in operating systems, for example Windows 98/NT and OS2, and its virus-scanning action is downloaded and executed when the operating system is being started. Some viruses, for example a Trojan Horse virus and a Time Bomb virus, which attacks specially designated information such as date or time, will attack the computer system or the PDA before the virus-scanning software is installed and stored in a system memory. Therefore, some unwanted changes might be made to computer components by a virus, such as a change to stored information or the making of propagation copies of the virus.